Stents are scaffolds which are positioned in diseased vessel segments to support the vessel walls. During angioplasty, stents are used to repair and reconstruct blood vessels. Placement of a stent in the affected arterial segment prevents elastic recoil and closing of the artery. Stents also prevent local dissection of the artery along the medial layer. Physiologically, stents may be placed inside the lumen of any space, such as an artery, vein, bile duct, urinary tract, alimentary tract, tracheobronchial tree, cerebral aqueduct or genitourinary stent. Stents may also be placed inside the lumen of non-human animals, such as primates, horses, cows, pigs and sheep.
In general, there are two types of stents: self-expanding and balloon-expandable. The balloon-expandable stent is placed in a diseased segment of a vessel by inserting an unexpanded stent into the affected area within the vessel. Prior to insertion, the stent is crimped onto a balloon which is inflated to expand the stent against the vessel wall. Inflation remodels the arterial plaque and secures the stent within the affected vessel. Balloon expandable stents may suffer from the collapse as a result of the natural elastic recoil of the vessel wall and lack of resilience of the stent itself.
Self-expanding stents are capable of expanding by themselves. There are many different designs of self-expanding stents, including helical, circular, cylinder, roll, stepped-pipe, high-order coil, braided wire, cage or mesh.
Self-expanding stents may be formed from super-elastic or shape memory metal. U.S. Pat. No. 6,013,854. In stents, nickel-titanium (NiTi) alloys are commonly used. http://mrsec.wisc.edu/Edetc/cineplex/NiTi/indexhtml, April, 2009. Stents formed from NiTi alloys are highly resilient, even when compressed because of the superelastic properties of the alloy. When cooled below the transformation temperature such as by liquid nitrogen, the NiTi alloy transforms to a martensite phase, holding a new shape until it warms back-up. This transformation can be referred to as a change between a martensite phase (stable at low temperatures) and an austenite phase (stable at high temperatures). http://en.wikipedia.org/wiki/Shape_memory_alloy, April, 2009.
The self-expanding stent is placed in the vessel by inserting the stent in a compressed state into the affected region, e.g., an area of stenosis. Compression or crimping of the stent can be achieved using crimping equipment (see, http://www.machinesolutions.org/stent_crimping.htm, April, 2009). The stent may also be compressed using a tube that has a smaller outside diameter than the inner diameter of the affected vessel region. Once the compressive force is removed or the temperature raised, the stent expands to fill the lumen of the vessel. When the stent is released from confinement in the tube, the stent expands to resume its original shape, in the process becoming securely fixed inside the vessel against the wall.
Each of the various stent designs that have been used with self-expanding stents has certain functional problems. For example, because the helical windings forming the stent terminate unevenly, the last portion of winding expands at a different rate from other areas in the stent. This problem, differential expansion, may be solved, in part, by introducing a transition zone between the main body of the stent and the end zone. See, e.g., U.S. Pat. Nos. 6,878,162, 6,969,402 and 7,169,175 for examples of various transition zones. However, because the transition zone joins two different, structural segments of the stent, an end zone on one side and a main body or helical portion on the other, the strut lengths of the transition zone vary. Such a variation can result in a change in stress or bending moment of the stent across the transition zone. This lack of uniformity affects the ability of the stent to be uniformly compressed and expanded which in turn impacts insertion of the stent into a convoluted vessel. Accordingly, there is a need to develop geometric designs which allow for uniform expansion across the transition zone, while still permitting maximal flexibility.
The present invention provides a geometric design for a stent that has both a high degree of flexibility and significant radial strength as well as provides for uniform expansion across the transition zone. The design of this stent also allows it to be inserted into small diameter vessels having complex geometry. The stent is further capable of responding dynamically to changes in blood pressure.